Liquid crystal screen displays possess advantages such as thinness, light weight, and applicability to low-voltage driving, and therefore, they have been used in many display applications including wrist watches, desk-top calculators, personal computers and word processors as well as in light shutter applications.
The most popular liquid crystal screen displays are the Twisted Nematic (TN) mode liquid crystal screen displays in which a liquid crystal layer is held between a pair of substrates and electrodes are placed on the respective substrates, for establishing an electric field for driving liquid crystal molecules in the liquid crystal layer. The TN mode liquid crystal screen displays do not have a sufficiently wide angle of view. Up to now, there have been proposed various driving mode liquid crystal screen displays having wider angles of view than the TN mode liquid crystal screen displays, these displays including the IPS (In-Plane Switching) mode in which a pair of electrodes for driving liquid crystal molecules are placed within the same pixel on the same substrate; the PVA (Patterned Vertical Alignment) mode in which a pair of substrates are alternately provided with a substantially linear electrode within the same pixel; the OCB mode (Optically Compensated Birefringence) mode which has an electrode layout similar to that of the TN mode but provides wider angles of view; and the MVA (Multi-domain Vertical Alignment) mode.
FIG. 17 shows one example of array substrates for use in the IPS mode liquid crystal screen displays. FIGS. 18a, 18b and 18c show sectional views of a liquid crystal screen display that uses the substrate shown in FIG. 17. In a pixel region defined by a pair of gate signal lines 4 and a pair of source signal lines 5 on an array substrate 2, pixel electrodes 6 and common electrodes 7 are both laid out in a comb-like fashion and placed with an insulating layer 11 between. The common electrodes 7 within one pixel region are electrically connected to those of the adjacent right and left pixel regions (in the drawing) by a common electrode line 8 integrally formed therewith. The source signal lines 5 formed on the same layer as the pixel electrodes 6 are connected to the pixel electrodes 6 through a thin film transistor (TFT) 9. The gate signal lines 4 are formed on the same layer as the common electrodes 7 and others to supply the TFT 9 with a signal for controlling the electrical connection between the source signal lines 5 and the pixel electrodes 6. In the region where the pixel electrodes 6 and the common electrode line 8 are overlapped, a storage capacitor 23 is formed.
An overcoat film (passivation film) 12 is laid over the surface of the array substrate 2 having these signal lines and electrodes formed thereon, and an alignment layer 16 is further formed so as to cover the overcoat film 12.
A liquid crystal screen display 1 comprises the array substrate 2 shown in FIG. 17 and an opposed substrate 3 which faces the array substrate 2 across a liquid crystal layer 18. On the surface of the opposed substrate 3 facing the array substrate 2, a lattice-like black matrix 14 for defining the pixel regions and a color filter 15 having sections corresponding to the pixel regions are formed and the alignment layer 16 is formed so as to cover them. Since the region immediately above the common electrode line 8 does not contribute to normal displaying by the pixels, a black matrix is sometimes used, in place of the color filter, in the region where the opposed substrate 3 faces the common electrode line 8.
In the formation of electrodes and signal lines on a substrate for liquid crystal screen displays, these elements are liable to electrical short owing to contamination with dust particles or pattern defects of the electrodes. Above all, array substrates for IPS mode liquid crystal screen displays are most likely to cause electrical short in their formation process because of the provision of comb-like pixel electrodes and common electrodes.
If the short-circuited portions are isolated by laser irradiation, the portions of the overcoat film which have been irradiated with the laser beams tend to be destroyed so that the signal lines and others reveal from the overcoat film. If the gate signal lines are exposed resulting from the laser irradiation, display unevenness is likely to occur in the regions repaired by the laser irradiation, when the liquid crystal screen display having such a substrate is continuously driven. In the case of a normally black display panel for instance, images displayed by the pixels in repaired regions are darker than those displayed by the surrounding pixels.
In fact, an IPS mode liquid crystal screen display was fabricated by use of an array substrate in which an overcoat film in the regions provided with gate signal lines had been removed by laser irradiation and was continuously driven at a temperature of 50° C. for twelve hours in a thermostatic oven. When images of intermediate gray scale are displayed by this display, display unevenness was observed in the regions which had been subjected to laser irradiation. This display unevenness is conceivably attributable to the fact that the voltage retention of the liquid crystal layer in the laser-irradiated regions locally decreases owing to local ion generation in the regions of the liquid crystal layer surrounding the laser-irradiated regions or to impurity ions which have been included in the liquid crystal layer beforehand and adsorbed in the laser-irradiated regions. In the regions where the overcoat film is destroyed, the signal lines having a specified potential are exposed. Therefore, electrons are introduced into the liquid crystal layer from the signal lines or the like and liquid crystal molecules are decomposed or electrically charged, resulting in ion generation or ion adsorption in the regions. The local uneven distribution of ions causes a drop in the voltage retention of the liquid crystal layer of these regions and, in consequence, display unevenness. Since the so-called reverse driving for alternately reversing the polarities of the pixel electrodes relative to the common electrodes is generally adopted, exposed elements generate ions in a certain period and retrieve ions in another period. Accordingly, there arises no serious problems even if the overcoat film in the regions where the pixel electrodes, the common electrodes and the source signal line are formed is destroyed. In contrast with this, if the overcoat film is destroyed in the regions where gate signal lines are formed whose polarity is negative with respect to the pixel electrodes and the common electrodes, negative ions are substantially constantly generated or positive ions are adsorbed in these regions. Therefore, ion concentration becomes extremely high in these regions, compared to other regions. Similar display unevenness occurs, for example, in cases where pin holes are created in the overcoat film laid over the gate signal lines or where the gate signal lines have level differences.
To restrict the influence of pinholes, Japanese Patent Publication (KOKAI) No. 10-206857, for example, proposes that the overcoat film be 0.4 μm or more thicker than the electrodes in contact with the overcoat film. According to this publication, although the exposure of the electrodes to the liquid crystal layer through pin holes can be reduced by use of an overcoat film having sufficient thickness, it has no effect of restraining the exposure of signal lines and others due to the destruction of the overcoat film by laser irradiation.
Japanese Patent Publication (KOKAI) No. 10-186391 has proposed that part of the electrodes be formed in contact with an alignment layer and the resistivity of the liquid crystal material be 1013Ω or more in order to restrain display abnormalities due to d.c. components remaining in the overcoat film (i.e., insulating film) of an IPS mode liquid crystal screen display. By exposing the pixel electrodes, ions in the liquid crystal layer can be retrieved on the exposed surface. However, since the voltage of the pixel electrodes is retained only by the storage capacitor during most of the period of time when the panel is driven, active retrieval of ions results in a drop in interelectrode voltage. More concretely, the ion concentration of the liquid crystal layer cannot be effectively reduced. Selection of a liquid crystal material based on resistivity is not necessarily appropriate in view of prevention of image persistence and high-speed response.
In such a situation, there have been demands toward liquid crystal screen displays capable of effectively restraining occurrence of display unevenness attributable to ions which have been locally generated within the liquid crystal layer.